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New research exploring the relationship between urban development and hazardous weather finds that urban landscapes and man-made aerosols have the potential to amplify and steer storms towards cities.

Scientists from the US Department of Energy's Pacific Northwest National Laboratory reported that these man-made works could help create stronger gusts, larger hail, and storms hitting cities sooner.

Illustrating Man-Made Impact on Weather Conditions

Atmospheric scientist Jiwen Fan modeled two thunderstorms - one situated near Houston, Texas, and another in Kansas City, Missouri. This experiment demonstrated separate and synergistic effects of the two anthropogenic activities and products on storms, hail, and rain.

In the first thunderstorm set for Houston, Texas, a supposedly gentle thunderstorm saw developed stronger and longer-lasting rainfall. Also, the storm was found to form earlier. Meanwhile, the Kansas City thunderstorm had an increase of about 20 percent in the frequency of its large hails, attributed to both urban land and aerosols.

Jiwen Fan shared her findings last December 1, at the American Geophysical Union 2020 fall meeting. She then answered queries from the public on Tuesday, December 15, in a virtual session.

"The novelty of our study is that we consider both urban land and aerosols together instead of their separate impacts," the atmospheric scientist noted. Previous works have demonstrated that urban land shapes weather, because of both its topological features and the heat it generates. Cities are generally warmer compared to natural landscapes as buildings absorb and retain the sun's heat, as well as blocking natural wind flow. Studies often examined the effect of urban landscape and aerosols - particles suspended in the atmosphere, common in spray products - separately, instead of observing their joint effect.

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Simulated Storms: A Model on Changing Hazardous Weather

In Fan's study, she modeled distinctly varying types of storms. Kansas City saw a violent, rotating, and hail filled thunderstorm. On the other hand, Houston was simulated with a gentler thunderstorm induced by sea breeze systems. Her simulations also included multiple iterations that isolate each variable, such as cities and aerosols' presence to observe their impact on the overall weather situation independently.

There was a significant increase in afternoon showers as both urban land and aerosols synergized in amplifying thunderstorm rainfalls for her Houston setup. Compared to the version of simulations with no cities installed, Houston was drenched in rainfall almost 30 minutes earlier, with rainfall volumes up by about 1.5 millimeters. The presence of human-developed landscapes also led to stronger winds. Additionally, the simulations revealed then when cool and denser air from the sea flowed inland toward Houston, it carried moisture. This cooler air met with the warmer, and less dense, city air. This creates a stronger convection compared to the city-less landscape.

Also, aerosols were a more noticeable factor in enhancing precipitation compared to urban land for the Houston simulations. Ultrafine particles transformed into cloud droplets as mixed-phase clouds and convection grew stronger. The presence of these particles amplified water vapor to cloud condensate conversions, making the resulting storm stronger.

"The aerosol effect really depends on the background concentration," Fan explains. She noted that for an already-polluted environment, the effect of adding aerosols may not be as significant as it does in a previously clean area.

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